When I made Minecraft fullscreen(not F11, but next to the X) while it was loading a world, the displayed section of Minecraft was the size of the small window. The window, however, did show fullscreen. Taking a screenshot only got an image of the small window, however.

What I expected to happen was...:
Minecraft to go fullscreen
What happened was...
The window full screened, but Minecraft did not.
How to reproduce...
1: Load a world
2: Fullscreen while it is loading

Making the window not fullscreen and fullscreen again fixed this.